ckeditor_custom_heading.module

<?php
use Drupal\ckeditor_custom_heading\Plugin\CKEditor5Plugin\Heading;

function ckeditor_custom_heading_ckeditor5_plugin_info_alter(&$plugins) {
    $def=$plugins['ckeditor5_heading']??null;
    if (!$def) return;
    $def->setClass(Heading::class);
}